This invention relates to a vacuum airtight element having a metal film electrode structure arranged in a vacuum airtight envelope, such as a field emission cathode used as an electron source for a display device or the like, and more particularly to a vacuum airtight element wherein at least a part of an electrode structure is formed of niobium nitride (NbN).
A field emission element having a field emission cathode incorporated therein has been conventionally known as one example of a vacuum hermetic element in the art. Now, a conventional vacuum airtight element will be described with reference to a field emission element by way of example.
When an electric field set at about 10.sup.9 (V/m) is applied to a surface of a metal material or that of a semiconductor material, a tunnel effect permits electrons to pass through a barrier, resulting in the electrons being discharged to a vacuum even at a normal temperature. Such a phenomenon is referred to as "field emission" as conventionally known in the art and a cathode constructed so as to emit electrons based on such a principle is referred to as "field emission cathode".
Recent development of fine processing techniques for a semiconductor permits a field emission cathode to be formed into a size as small as microns. The development permit number of such field emission cathodes to be formed on a substrate, resulting in providing a field emission element of the surface emission type. It is proposed that the field emission element thus provided is used as an electron source for a display device, a CRT, an electron microscope, an electron beam device.
Such a conventional field emission element will be more detailedly described hereinafter with reference to FIGS. 7 and 8. The field emission element includes a field emission cathode formed on a glass substrate 101. Application of the field emission element to a display device is carried out by arranging the glass substrate 101 in a manner to be opposite to a phosphor deposited anode substrate of a transparent glass material and spaced therefrom at a predetermined interval, to thereby form an airtight envelope, which is then evacuated to a high vacuum.
The field emission cathode formed on the glass substrate 101 is constructed of stripe-like cathode line electrodes 102 formed by sputtering or the like, a resistive layer 103 formed on the cathode line electrodes 102, a plurality of emitter cones 106 formed on the resistive layer 103, and gate line electrodes 105 formed in proximity to the emitter cones 106 so as to surround a tip end of each of the emitter cones 106, resulting in being a field emission array of the Spindt-type.
The above-described resistive layer 103 is laminatedly formed thereon with an insulating layer 104, on which the gate line electrodes 105 are then formed.
The emitter cones 106 may be formed while keeping a pitch between each adjacent two of the emitter cones 106 at a level as small as 10 microns or less, so that tens of thousands to hundreds of thousands of such emitter cones thus formed are arranged on one glass glass plate 101. In the field emission element thus constructed, a distance between a gate and a cathode is kept at a level as small as a sub-micron, so that application of a voltage V.sub.GE 108 as low as tens of volts between the gate and the cathode permits electrons to be emitted from the emitter cones 106.
Application of the field emission element to a display device is carried out in such a manner that the anode substrate arranged opposite to the glass substrate 101 is formed thereon with an anode electrode, on which a phosphor is laminatedly deposited in a dot-like pattern.
Thus, application of a positive voltage to the anode electrode causes electrons emitted from the emitter cones 106 to be captured by the anode electrode, so that the electrons impinge on the dot-patterned phosphor laminated on the anode electrode, resulting in exciting the phosphor, leading to luminescence of the phosphor. The luminescence may be observed through the transparent anode substrate.
The emitter cones 106 are arranged on the above-described plural cathode line electrodes 102 arranged in a stripe-like manner on the glass substrate 101 and the plural gate line electrodes 105 are arranged in a stripe-like manner and so as to extend in a direction perpendicular to the cathode line electrodes 102.
Thus, the stripe-like cathode line electrodes 102 and stripe-like gate line electrodes 105 cooperate with each other to define a matrix, which is scanned by a cathode scanning section (not shown) and a gate scanning section (not shown).
This causes electrons to be selectively emitted from the emitter cones 106 depending on an image signal, resulting in a corresponding portion of the dot-patterned phosphor emitting light, so that an image may be displayed on the anode substrate.
In this instance, for example, an image signal is applied to the gate scanning section, so that an image for one sheet is displayed on the anode substrate when scanning for one field terminates.
In the conventional field emission element, the cathode line electrodes 102 and gate line electrodes 105 are formed on the glass substrate 101 and insulating layer 104 by sputtering or the like, respectively. In general, the cathode line electrodes 102 and gate line electrodes 105 each are made of a high-melting material such as niobium (Nb), molybdenum (Mo), tantalum (Ta), tungsten (W) or the like. Thus, the conventional flied emission element causes disadvantages.
For example, in the conventional field emission element, a thin film 111 of high-purity niobium is formed on the glass substrate 110 by sputtering as shown in FIG. 9. Then, the film. 111 is subject to dry etching such as RIE or the like, to thereby be formed into a stripe-like configuration, resulting in the cathode line electrodes 102 being formed. However, this causes etching marks to be left on the glass substrate 110, so that it is required to remove the marks by somewhat dissolving a surface of the glass substrate 110 by a depth .delta. shown in FIG. 10 by means of hydrofluoric acid.
Unfortunately, this causes hydrofluoric acid to enter an interface between the glass substrate 110 and the thin film 111 of high-purity niobium because the thin film 111 of Nb fails to exhibit satisfactory bond or adhesion strength to the glass substrate 110, leading to peeling of the thin film 111 of Nb from the glass substrate 110, resulting in a film peel region occurring as shown in FIG. 10.
Such a film peel region causes performance or characteristics of the field emission element to be highly deteriorated, leading to a decrease in yields of the element.
Also, the thin film of high-purity niobium is readily subject to oxidation, to thereby form niobium oxide (NbO.sub.2). The niobium oxide is decreased in etching speed as compared with niobium. Unfortunately, this causes an important disadvantage of the field emission element.
More particularly, supposing that a thin cathode film 121 for cathode line electrodes is formed on a glass substrate 120 and then an insulating layer 122 of SiO.sub.2 and a thin film 123 of Nb are formed on the cathode film 121 in order, a surface 124 of the Nb film 123 is oxidized by oxygen contained in an ambient atmosphere and a boundary surface of the Nb film 123 bordering the insulating layer 122 is oxidized by oxygen contained in SiO.sub.2.
Then, when etching is carried out on the Nb film 123 to provide the Nb film 123 with openings, to thereby form the Nb film 123 into gate line electrodes, the Nb film 123, as shown in FIG. 12, is etched in such a manner that holes are bored in a central region of the film 123, resulting in the openings each being formed into a barrel-like shape in section because niobium is increased in etching speed as compared with niobium oxide.
Formation of such a barrel-like opening renders fine patterning difficult, so that an increase in degree of integration is failed.
In general, a thin film of Nb is readily subject to oxidation, to thereby cause an increase in surface charging. Thus, in the conventional field emission element, when the gate line electrodes are oxidized, an electric field formed between the gate like electrodes and the emitter cones is decreased, so that the emitter cones fail to satisfactorily emit electrons.